Caption: Earthlike alien planet. Artwork of the view from the surface of a moon orbiting an Earthlike alien planet (upper right). A planet's orbit and its size have to fit within a narrow range for it to have a surface temperature suitable for liquid water, and for it to retain an atmosphere. Also seen here are nebulae (red), clouds of gas and dust where new stars may form, such as those of the star cluster seen on the horizon. A comet is at centre left. As of 2010, hundreds of planets have been detected around other stars.